Part 89

a, vestibule; b, cochlea; c, external semicircular canal; d, superior semicircular canal; e, posterior semicircular canal; f, scala tympani; g, scala vestibuli; h, fenestra cochleæ; i, fenestra vestibuli.]

The labyrinth (Fig. 173) consists of a cavity, the =vestibule= (a), and connected with this a coiled tube, the =cochlea= (b), and the three =semicircular canals= (c, d, and e).

The =cochlea= (b) is situated in the promontory,--the rounded elevation of the petrous bone extending craniad from the fenestra cochleæ. It consists of a bony tube coiled spirally about a central column known as the =modiolus=. The coil has nearly the form of a snail’s shell, and is somewhat more lengthened in the cat than in man. A plate of bone, the =lamina spiralis=, incompletely divides the cavity of the canal lengthwise into two parts, known as =scalæ= (f and g). One of these opens at the fenestra cochleæ (h) into the tympanic cavity (or would so open but for the thin membrane covering the fenestra); this is known as the =scala tympani= (f). The other scala is connected with the vestibule (a) and is known as the =scala vestibuli= (g).

The =vestibule= (a) is a pyramidal cavity situated mediad of the caudal end of the cochlea. It is connected with the scala vestibuli of the cochlea, and with the three semicircular canals. The fenestra vestibuli (i) is an opening in its bony wall, filled in the recent state by the stapes. On the medial surface of the vestibule are a number of minute orifices through which the auditory nerve enters.

The =semicircular canals= (c, d, and e) are three curved tubes, each forming about two-thirds of a circle, and opening at both ends into the vestibule. They are imbedded in the petrous bone. The planes of the three tubes intersect nearly at right angles; from their position in man they have been named =external= (c), =superior= (d), and =posterior= (e). The external one (c) is nearly horizontal in position, and surrounds the curved fossa (Fig. 25, f) for the stapedius muscle, caudad of the fenestra vestibuli. The superior (Fig. 173, d) and posterior (e) are in nearly dorsoventral planes, but at right angles to each other. The superior (d) lies in the ridge forming the caudal boundary of the appendicular fossa. The posterior (e) is situated in a continuation caudad of the ridge which forms the dorsal boundary of the internal auditory meatus. One end of each canal is slightly enlarged to form an =ampulla=. The superior (d) and posterior (e) canals join at one end and enter the vestibule by a common opening. One end of the external canal (c) opens into the vestibule so close to one end of the posterior canal (e) that they appear to have a common opening, and this is commonly stated to be the case; according to Jayne, however, the openings are really separate. The two remaining ends (of the superior (d) and external (c) canals) also enter the vestibule separately, so that the three canals enter the vestibule by five openings.

III. THE OLFACTORY ORGAN. ORGANON OLFACTUS.

The olfactory organ consists of the olfactory mucous membrane, in the dorsocaudal part of the nasal cavity. The nasal cavity has been described in connection with the respiratory system.

IV. THE ORGAN OF TASTE. ORGANON GUSTUS.

The sense of taste is located in the mouth-cavity, especially in the mucosa of the tongue and the soft palate. The proper organs of taste are the so-called taste-buds, found on the papillæ of the tongue, the soft palate, etc. Their exact distribution in the cat has apparently not been worked out.

V. THE INTEGUMENT. INTEGUMENTUM COMMUNE.

The skin of the cat consists of the usual two layers, the outer =epidermis= and the inner fibrous =corium=. The skin is particularly thick and tough about the sides of the neck and face.

The skin is covered almost completely with fine soft hairs. Only the end of the nose, the pads on the soles of the feet, and the nipples are without hair. A number of long stiff sensory hairs (vibrissæ) are found on the upper lip, the cheek, and above the upper eyelid.

The pads on the soles of the feet are cushion-like projections of skin, composed largely of connective and elastic tissue and fat. There is one large pad for each foot, and in addition a small pad for each digit. The fore limb has besides a small conical pad which is situated almost exactly over the pisiform bone.

The =nails= are horny outgrowths of the epidermis, covering the distal phalanges.

The =muscles= of the skin are the cutaneus maximus and platysma; these have been described (page 93). The superficial facial muscles are differentiations of the skin-muscles.

APPENDIX.

PRACTICAL DIRECTIONS.

The following appendix on the dissection and study of the cat is not intended as a general treatise on methods of preservation and dissection, but attempts only to give the application of well-known methods to the dissection and study of the particular animal with which the book deals. General methods which are referred to but not described must be looked up, if unfamiliar, in the manuals of general methods.

Aids to Dissection and Study of the Cat.--A copy of Jayne’s “Mammalian Anatomy” should be in the laboratory for reference. Only the volume on the Bones of the Cat has been issued thus far.

Wilder and Gage’s “Anatomical Technology” will be found very useful for methods of work.

The plates of Strauss-Durckheim’s “Anatomie du Chat” are exceedingly valuable for the bones and muscles, and should be in the laboratory if possible. Outline reproductions of these plates have been published by H. S. Williams (G. P. Putnam’s Sons, New York, 1875). The use of both the original plates and the reproductions is made difficult by the antiquated nomenclature used for the structures figured.

Method of Use of the Present Directions.--Except in the case of the Bones, the order of study of the structures is usually not the same as the order in which they are described. This is of course due to practical difficulties in dissection. The present directions give the order of study, or at least the order of dissection; of course after dissection the structures can be reviewed in the logical order given in the descriptions. But in the first study and dissection, the student is to use these directions for the order of work, while using the body of the book for the descriptions. It will be necessary therefore to keep the book opened in two places,--one in the appendix, for the directions; one in the body of the book, for descriptions. In studying the muscles, for example, the student will follow the order of dissection given in the appendix, but as every muscle is dissected he will find, confirm, and study the description of that muscle given in the body of the book. Other organs are studied in a similar manner.

Before commencing the study, read that portion of the preface which explains the use of terms of direction (page VII).

Note that the page on which any figure is found may be determined by consulting the list of figures given in the Table of Contents.

THE BONES.

The bones will of course be studied from dried specimens. These may be prepared once for all and kept in the laboratory from year to year. A number of mounted skeletons of the cat should be at hand; these must be prepared by some one who has skill in such work.

Preparation of Separate Bones.--For preparing the separate bones the liquid-soap process recommended by Wilder and Gage (“Anatomical Technology,” p. 107) will be found most convenient. This is as follows: The skin and soft parts are removed as far as possible without injury to the bones.

Make the following “liquid-soap” mixture: